The ontogenesis of the ERG is often correlated with the appearance of photoreceptor outer segments and rhodopsin. This has been studied in species where light impinges upon the retina during development. Primates, however, show visual function at birth, developing competent outer segments intrauterine and in the dark. The larval lamprey forms outer segments (without rhodopsin) in the dark under a dense dermal cornea and the transformed maturing lamprey shows visual function when the cornea clarifies. The pigment epithelium changes markedly over this same period. The post-natal influence of light on retinal integrity and pathogenesis is profound. It is likely to also have a significant influence on the normal development of the retina. Thus, the lamprey retina is a potentially useful natural model for primate, and human, retinal development and for developmental anomalies and dystrophies of the retina. The goal of this research is to correlate the morphological and electrical events of retinal development during the transformation of the lamprey, Petromyzon marinus, a cold-blooded vertebrate. Its eye is easily maintained out of the body for electrophysiology and is of reasonable size for analysis by light and electron microscopy. By means of these methods, Golgi impregnations and HRP uptake, we will trace the development of the cellular and synaptic organization of the lamprey retina. The formation and turnover of outer segment discs will be studied by radio-labelled amino-acid injection and radioautography. During transformation and the development of the retina, these observations will be correlated with the ERG and the appearance of photolabile pigments.